Thrombosis is the formation of a hemostasis or a blood clot inside a blood vessel. Thrombosis in coronary artery of the heart or cerebrovascular part causes a heart attack or cerebral infarction. Thrombosis referred to as silent killer is becoming the main cause of death for our contemporaries. The problem is that thrombosis or bleeding disease is not caused by genetic defect only and cause thereof is not discovered clearly.
More seriously, thrombosis prevalence rate is fast increasing due to genetic defect and acquired factors. Therefore, an apparatus and method have been required to make a quantitative test for thrombosis or bleeding disease and to carry out an early diagnosis and prognosis decision.
There are many factors which play roles in hemostasis at vascular injury sites. All of biochemical and biological mechanism of each factor play a critical role and hemostasis of a platelet plays the most crucial role. A platelet is not attached to the arterial walls at no injury sites, but biochemical and biological mechanism is activated at vascular injury sites, thereby hemostasis is achieved regardless of flow conditions.
Many method and devices have been developed to subdivide and test a platelet function. A platelet function test is an important test to discern bleeding disorders which are caused by a congenital or acquired disorder of platelet function in case of the bleeding disorders having no numerical platelet disorder. Also, this platelet function test is being used to examine an increase of hemorrhage tendency or a drug-tolerance which is caused by antiplatelet agent used for the treatment or prevention of cardiovascular disorders.
If there is an injury at endothelial cells in the blood vessel, an inner material in the endothelial cells such as collagen is exposed to the blood and a platelet is attached to the material and is activated. Attachment mechanism of platelet has different characteristics depending on blood flow conditions.
In particular, if a blood flow rate is high in the artery and a shear stress applied to the blood vessel wall is high, platelet is not attached to the inner membrane of the blood vessel easily. In this condition, von Willebrand factor (vWF) is activated and is easily attached to the wall of the blood vessel and a platelet is attached to the wall of the blood vessel by vWF. Of course, it is known that a glycoproteic receptor complex such as GPIb-IX-V which is contained in the platelet cell membrane induces the reaction with vWF, leading to the attachment.
As such, an attached platelet induces an aggregation by attracting the same kind of platelet and leads to hemostasis. Then, hemostasis is reinforced by fibrin.
However, the function of platelet is not always favorable, but can produce adverse effect in a certain flow condition or situation. For example, when the blood vessel wall became narrow locally by artery hardening, a platelet passing through this narrow part is exposed to a high shear ratio and is activated, and then adhesion and aggregation occurs in the rear of the narrow part and leads to thrombosis which blocks the blood vessel.
As explained above, a platelet and vWF are activated by the size of a blood flow, i.e., a shear stress due to the flow, thereby leading to the increase of an adhesive property and the generation of hemostasis. It is known that the shear stress required for the activation of a platelet or vWF is at least 8 Pa and the shear rate is at least 5,000 l/s.
As such, various devices are suggested and developed for an early diagnosis and prognosis test of hemostasis or bleeding disease and they can be divided into an electric method, an optical method, a time measuring method to stop bleeding, etc., based on the measuring sensor.
A bleeding time (BT) method was developed about 100 years ago which measures a bleeding time and is still being used for a screening test of a platelet function. However, a current platelet function test has problems such as a difficult standardization, a low diagnostic validity and a use of invasive technique and therefore an objectified measuring method to measure a platelet function has been required.
To solve the problems, a platelet function analyzer (e.g.: PFA-100) is developed which is being used to measure a platelet function. This analyzer uses a feature that a platelet is aggregated by activated vWF in a high shear rate. To measure this feature, after whole blood flows in a long capillary tube, a platelet aggregates in an orifice coated by ADP or epinephrine together with collagen and then time by which the orifice is blocked is measure by pressure, flow rate, etc.
This platelet function test strictly depends on the function of vWF. This test has disadvantages that the test depends on hematocrit (Hct) and an antiplatelet test such as a test using aspirin or clopidogrel is not possible. Also, disadvantageously, two step tests are required for the function test of platelet and cost for the test is increasing.
In particular, to activate vWF, a blood sample must be exposed for a predetermined time with a high shear rate. For this, PFA-100 uses a method for making blood flow at high speed in a long capillary tube. However, disadvantageously, this method requires a large quantity of blood. Also, it is a disadvantage that vWF is not activated at the center of the capillary tube where a shear rate is minimum while vWF is activated near the tube wall where a shear rate is maximum, thereby a repeatability of test results is not guaranteed.
IMPACT of Diamed Co., Ltd. uses a rotation-type Couette flow in the form of Cone-Plate. In this method, a uniform shear stress is applied to the blood contained therein and an attachment degree of platelet is measured when a high shear stress is applied. Like PFA-100, this method has a disadvantage that it depends too much on the concentration and function of fibrinogen and vWF.
Verify-NOW (Accumetrics) uses a method to measure an aggregation degree of platelet by a turbidity using an optical sensor. In this method, an agonist is mixed with blood and then a microbead on which collagen is coated is reacted so that a platelet in the blood is aggregated. Then, turbidity is measured over time. The frequency of use this method is being increased recently, but this method still has disadvantages of prior methods to measure a turbidity.